In a QAM symbol transmission system of the type disclosed herein, in-phase "I" and quadrature phase "Q" signal components modulate respective cosine and sine carriers. A data symbol being transmitted is represented by both I and Q components. Each symbol may represent several bits, and the number of bits/symbol dictates the type of QAM system, i.e., 16 QAM, 32 QAM, etc. Each symbol is mapped into a prescribed location in a four quadrant grid-like constellation using a look-up table, with a prescribed number of symbols occupying assigned areas in each quadrant. Thus in a 32 QAM system each quadrant of the constellation contains 8 symbols at prescribed locations with respect to in-phase and quadrature phase I and Q axes. A form of differential coding may be used whereby certain bits designate the constellation quadrant in which the symbol is located, and certain bits designate the particular point in that quadrant assigned to the symbol. QAM systems of the type described are well known.
At a receiver, the QAM modulated carrier frequency may be offset relative to a carrier reference signal locally generated at the receiver. This carrier frequency offset is typically manifested by a dynamic rotation of the QAM constellation recovered at the receiver, and may be significantly reduced or eliminated by means of a phase locked loop (PLL) network. The PLL generates a frequency/phase correction signal for dynamically restoring (derotating) the QAM constellation to a correct orientation with the symbols in expected locations. A second order PLL is often preferred for correcting the described carrier offset since a second order PLL exhibits a wide tracking range for correcting a wide frequency deviation, e.g., up to a 60 KHz deviation for example, and typically is able to correct such deviation completely.
The stability of the PLL is a function of both loop delay as determined by the number of clock cycles in a digital phase locked loop (DPLL), and loop gain. High loop gain, is generally desirable but often degrades the stability of the loop. In addition, for a given gain, loop stability decreases as the number of clock cycles (delays) increases. Apparatus according to the present invention addresses this compromise involving loop delay and loop gain in a carrier recovery system.